linux-kernel - Re: [PATCH v2] csky: fix csky_cmpxchg

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Message-ID: <CAJF2gTS0bm_PSSJrXrzaL98zeJt-nguj85Dzg6zNG6Qy_JD+Vg@mail.gmail.com>
Date: Mon, 28 Oct 2024 10:54:05 +0800
From: Guo Ren <guoren@...nel.org>
To: yang li <yang.li85200@...il.com>
Cc: linux-csky@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] csky: fix csky_cmpxchg_fixup not working

Hi yang,


On Mon, Oct 21, 2024 at 3:51 PM yang li <yang.li85200@...il.com> wrote:
>
> Hi Guo Ren:
> In C language, your conclusion is correct, but in assembly language,
>  global_symbol != &global_symbol
> I did the following experiment:
>
> liyang@...ang-virtual-machine:~/Desktop/test$ ls
> main.c  test.s
> liyang@...ang-virtual-machine:~/Desktop/test$ cat test.s
>
> .globl test_symbol
> test_symbol:
>         nop
> liyang@...ang-virtual-machine:~/Desktop/test$ cat main.c
> #include <stdio.h>
>
> extern unsigned long test_symbol;
> int main(void)
> {
>         printf("test_symbol = 0x%lx\n",(unsigned long)test_symbol);
>         printf("&test_symbol = 0x%lx\n",(unsigned long)&test_symbol);
>
>         printf("main = 0x%lx\n",(unsigned long)main);
>         printf("&main = 0x%lx\n",(unsigned long)&main);
> }
> liyang@...ang-virtual-machine:~/Desktop/test$ gcc main.c test.s --static -o test
> liyang@...ang-virtual-machine:~/Desktop/test$ ls
> main.c  test  test.s
> liyang@...ang-virtual-machine:~/Desktop/test$ readelf test -s | grep test_symbol
>    884: 000000000040170c     0 NOTYPE  GLOBAL DEFAULT    7 test_symbol
> liyang@...ang-virtual-machine:~/Desktop/test$ readelf test -s | grep main -w
>   1605: 0000000000401685   135 FUNC    GLOBAL DEFAULT    7 main
> liyang@...ang-virtual-machine:~/Desktop/test$ ./test
> test_symbol = 0x4b853001f0f90
> &test_symbol = 0x40170c
> main = 0x401685
> &main = 0x401685
>
> The above test can lead to the conclusion that:
> Both c_symbol and &c_symbol represent the address of a symbol, but
> &ASM_symbol represents the address of a symbol while ASM_symbol
>  represents the opcode stored at that address.
>
> On Thu, Oct 17, 2024 at 2:05 PM Guo Ren <guoren@...nel.org> wrote:
> >
> > On Wed, Oct 16, 2024 at 5:56 PM Yang Li <yang.li85200@...il.com> wrote:
> > >
> > > In the csky_cmpxchg_fixup function, it is incorrect to use the global
> > >  variable csky_cmpxchg_stw to determine the address where the exception
> > >  occurred.The global variable csky_cmpxchg_stw stores the opcode at the
> > >  time of the exception, while &csky_cmpxchg_stw shows the address where
> > >  the exception occurred.
> > >
> > > Signed-off-by: Yang Li <yang.li85200@...il.com>

You convinced me. Applied to csky/linux-next, thanks!


> > > ---
> > > V1 -> V2:Eliminate compilation warnings
> > >
> > >  arch/csky/mm/fault.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/csky/mm/fault.c b/arch/csky/mm/fault.c
> > > index a885518ce1dd..5226bc08c336 100644
> > > --- a/arch/csky/mm/fault.c
> > > +++ b/arch/csky/mm/fault.c
> > > @@ -45,8 +45,8 @@ static inline void csky_cmpxchg_fixup(struct pt_regs *regs)
> > >         if (trap_no(regs) != VEC_TLBMODIFIED)
> > >                 return;
> > >
> > > -       if (instruction_pointer(regs) == csky_cmpxchg_stw)
> > > -               instruction_pointer_set(regs, csky_cmpxchg_ldw);
> > > +       if (instruction_pointer(regs) == (unsigned long)&csky_cmpxchg_stw)
> > > +               instruction_pointer_set(regs, (unsigned long)&csky_cmpxchg_ldw);
> > csky_cmpxchg_ldw(stw) is a label symbol, not a variable.
> >
> > arch/csky/kernel/atomic.S:
> > GLOBAL(csky_cmpxchg_ldw)
> > GLOBAL(csky_cmpxchg_stw)
> >
> > Your modification does not affect the ASM output.
> >
> > (gdb) p main
> > $1 = {void (void)} 0x5fa <main>
> > (gdb) p &main
> > $2 = (void (*)(void)) 0x5fa <main>
> >
> > >         return;
> > >  }
> > >  #endif
> > > --
> > > 2.34.1
> > >
> >
> >
> > --
> > Best Regards
> >  Guo Ren



--
Best Regards
 Guo Ren